Troubleshooting Solvent Substitution Challenges in Coatings

TAGS:  Solvents 

1. Why Solvents are Unique?
2. Why Change Solvents?
3. What is the Solvent Doing in the Formulation?
4. How is the Formulation Applied?
5. Categories of Solvents
6. Replace Solvents with Similar Structure and Properties
7. Solvent Interactions with Other Solvents and Polymers
8. Solvent Evaporation is a Dynamic Process
9. Other Solvent Properties that Can Affect Performance
10. Check the Regulations of the New Solvent
11. Conclusion

Why Solvents are Unique?

Solvents are the Houdini’s of the coating formulation. These escape artists are the only ingredients in a coating formulation that are added in order to leave. The coating never reaches its optimal performance until the solvent has escaped.

How and when the solvent comes out often determines if you have a good coating or a poor coating. Application, appearance, and flow properties are all related to how the coating interacts with and releases solvent.

Let's understand the need to change solvents and their role in the formulation.

Why Change Solvents?

It is a lot of trouble to change a solvent, so why do we do it?

• Often regulations dictate a change in solvents.
• Sometimes new government tests or customer screening have determined whether the solvent is a hazard, toxin, or just suspect.
• Maybe a new label requirement of a “dead fish” or an “exploding chest” makes change appropriate.
• Perhaps, limited solvent supply has increased costs.
• Occasionally the customer just doesn’t like the smell.

Whatever the reason, solvent changes are often time-consuming and expensive to implement.

What is the Solvent Doing in the Formulation?

Solvents play several different roles in a coating formulation. A solvent is primarily used to help either dissolve or disperse the paint ingredients. A solvent can lower the viscosity and allow the coating to be applied. However, solvents are added to formulations for many other reasons.

When replacing a solvent, the first question to ask is:


Multicomponent and complicated coating formulations make it difficult to understand what the solvent is doing. Often, the solvent being replaced is one of several in the formulation.

Formulators are notorious for adding their ingredients to old formulations without documenting why. New formulators are hesitant to take something out because they don’t know why it was added initially. All this adds to formulation complexity. This is particularly true of solvent blends in formulations. Often why a solvent was added isn’t obvious until the solvent is removed from the formulation.

→ Learn How to Formulate Waterborne Coatings by Understanding Different Techniques to Meet Low VOC Regulations

How is the Formulation Applied?

Different solvents affect properties depending on the coating application.

• Is the formulation spray, roll, or brush applied?
  Solvents in formulations are optimized based on the application method. Obtaining the correct viscosity at the shear rate of the application often depends on selecting the correct solvent. Solvents contribute to film appearance and flow depending on the temperature of the application.


• Is the formulation a baked or air-dry system?
  The evaporation rate of the solvents is dependent on the temperature of the paint during application and cure. Much slower evaporating solvents are used in baked systems than in air-dry systems. Because of the higher temperature, slower solvents are often required in high bake coil coating systems, while faster solvents are often used in ambient air-dry systems. 

Categories of Solvents

The most common way of categorizing solvents is by molecular structure. The three broad categories are:

• Hydrocarbon solvents include both aromatic and aliphatic, such as xylene and hexane respectively. Hydrocarbon solvents, historically used for coatings, have primarily been replaced by more effective solvents that give lower VOC. They are still used where regulations allow because of their low cost. 
• Halogenated solvents include both chlorinated, like methylene chloride, and fluorinated. The advantage of these solvents is their non-flammability. Halogenated solvents have also been replaced due to toxicity concerns (chlorinated), Ozone Depleting Potential (ODP) concerns (fluorinated), or other regulatory and cost pressures.
• Oxygenated solvents, most commonly used in coatings, include alcohols, esters, glycol ethers, and ketones. However, combinations of these are very common, i.e. ester-alcohols like Texanol™ or glycol ether esters like PM acetate.

Because of their extensive use in all different types of coatings, for the remainder of the article, I will limit the discussion to oxygenated solvents.

Replace Solvents with Similar Structure

Try first to replace solvents with another of the same category and as similar in structure as possible. If the properties are not too different, sometimes just a small modification of the molecule won’t be noticed in the formulation.

• Replacing acetone with the molecule with one extra carbon, methyl ethyl ketone, might work.
• Maybe replacing ethyl acetate with methyl acetate will give an identical formulation.

Often, the key performance properties are just too different. It is either too fast, too slow, not compatible, or just smells differently.

Replace Solvents with Similar Properties

The formulator often needs to replace solvents based on similar properties rather than a similar structure. The two most common properties of solvents used in solvent selection are:

• Evaporation rate
• Solvent activity

Evaporation Rate

The evaporation rate of a solvent is a key measure of how fast (or slow) a solvent escapes from the coating. Coating formulators need to tailor the evaporation rate of the coating to the application method, whether the coatings are atomized into small particles during a spray operation or applied by brush.

How is Evaporation Rate Determined?

The relative evaporation rate (RER) of the solvent is usually indicated by comparison with another solvent. RER is the ratio of the time required to evaporate a measured volume of a reference solvent to the time required to evaporate the same volume of an unknown reference solvent. The RER for all solvents is measured in a special chamber under controlled environmental conditions:

• Temperature of 25°C
• Airflow at 21 liters/min, and
• Relative humidity at 0%

The time, in seconds, is measured for 90% of a known volume of the pure solvent to be evaporated.


The most common reference standard worldwide is n-butyl acetate which is set at 468 seconds. The ratio of the times is normalized to 1.0 by dividing the time for butyl acetate with the time for the other solvents.

• If the time in seconds for a solvent to evaporate is less than 468 seconds, the relative evaporation rate will be greater than 1.0, and this solvent will be “more volatile or faster” than butyl acetate.
• If the time for a solvent to evaporate is greater than 468 seconds, the relative evaporation rate will be less than 1.0, and this solvent will be “less volatile or slower” than butyl acetate.

Problems with Evaporation Rate of Solvent Blends

Because the relative evaporation rates are expressed as ratios of time, they are not linear and shouldn’t be averaged.

For example, say you wanted to use a blend of two ketone solvents with much different relative evaporation rates.

• The first is methyl ethyl ketone, much faster than butyl acetate, with an RER of 3.8.
• The other is methyl amyl ketone, much slower than butyl acetate, with an RER of 0.4.

If you mixed these in a 60:40 weight percent blend, the average relative evaporation rate is:


However, the actual RER of the blend is closer to 1.0. The blend has a much lower RER and is much slower than the average calculated RER. A computer program is often needed to calculate the correct evaporation rate of the solvent blend.

Another problem is that solvents that are very similar in structure can have very different evaporation rates. For example, methyl ethyl ketone (MEK) and tetrahydrofuran (THF) have the same molecular weight. Even though these molecules are isomers of one another, THF is 65% faster in evaporating than MEK. Often seemingly small changes in the structure of the molecule can greatly impact the evaporation rate. Isopropanol, with a branched structure, evaporates 70% faster than the linear alcohol, n-propyl alcohol.

Probably the biggest problem in calculating the relative evaporation rate of solvent blends is that solvents interact with other solvents in ways difficult to predict. Solvents often have either strong or weak attractive forces between each other. These forces can either speed up the evaporation rate of the blend, or slow it down. When you take into consideration the interactions between the solvents and the different polymers in the formulation, the problem becomes even more complex and difficult to predict. Some computer programs take these forces into consideration when calculating the evaporation rate of blends.

Solvent Activity: Classification and Determination

Solvent activity is also referred to as solvent power or solvency. The solvent activity of a particular solvent is dependent on what polymer you are trying to dissolve. One of the oldest ways of defining solvent activity was to group solvents in categories related to how they dissolved nitrocellulose resins, one of the first resins used in coating applications.

Solvents Classification Based on How They Dissolve Nitrocellulose Resins

Diluents: They had the lowest solvent activity for nitrocellulose resins. They did not interact to dissolve or reduce the viscosity of nitrocellulose and only acted as liquids to dilute the coating and take up space in the formulation. They are often added to reduce the cost of a formulation. Hydrocarbon solvents are most often classified as diluents.

Latent solvents: They are also referred to as cosolvents. Alone they are poor solvents for the nitrocellulose resin, but when mixed with another latent solvent or diluent, the mixture becomes an active solvent blend. Solvents in this category are primarily alcohols.

Active solvents: They dissolve resins and reduce their viscosity. They are most often similar in structure to the resins they dissolve and follow the adage; “Likes dissolve likes”. Since most coating resins contain oxygen in their structure (acrylic, epoxy, urethane, and cellulosic), oxygenated solvents tend to be more active and more expensive than hydrocarbon solvents.

Viscosity Reduction

One of the best ways for determining the solvent activity is to measure the viscosity of the solvent and resin pair. Since coatings are formulated in relatively concentrated solutions, using as little solvent as possible, these viscosities should be measured in similar concentrated solutions.

The most active solvents for coatings are those solvents that are more efficient in reducing the viscosity of a resin. Active solvents, ones with better “cutting” power, are used to minimize the VOC content of a coating. Most resin manufacturers supply information on whether their resin is soluble in different solvents. This “yes” or “no” designation is often of little help in determining a replacement solvent.

A better indication of comparative activity is to measure the actual viscosity of the solvent and resin at identical resin solids. Try to select a solvent that gives comparable viscosity reduction at the same resin concentration as the resin in your formulation.



Problem with Viscosity Reduction

Solvents interact with resin or polymer in a complicated dance often determined by many different interactions. These solvent-polymer interactions can result in one solvent dissolving the polymer producing a high viscosity solution, while a similar solvent causes the polymer to form a dispersion or aggregate that results in a lower viscosity.

While resulting viscosity is dependent on the concentration of the polymer in the solvent, solvent-polymer interactions in dilute solutions are often different in concentrated solutions. When comparing solvent activity by measuring viscosity differences, the viscosities should be at the same concentrations.

Balancing Evaporation Rate and Solvent Activity

Finding a solvent with a similar evaporation rate or similar solvent activity may mean that you find a completely different solvent structurally than the one you have been using. For example, xylene and dimethyl acetamide have the same evaporation rate, but are completely different solvents with very different solvent activity and will probably behave much differently in the formulation.

Solvent activity similarities in one polymer often don’t translate to a different polymer. For example, xylene and n-butyl acetate have very similar solvent activity in one polymer, equally reducing the viscosity of an acrylic copolymer of methyl methacrylate and butyl acrylate at 50% solids. But the solvent power of xylene and n-butyl acetate are very different when put into an epoxy resin. While n-butyl acetate is a good solvent for the epoxy, forming a low viscosity solution at 50% solids, xylene is insoluble in the epoxy.

Variation in solvent power in different resins makes solvent substitution particularly difficult in coating systems that contain several different resins or cross-linking agents.

Solvent Interactions with Other Solvents and Polymers

Often there are interactions between solvents and the additives in the formulation that impact stability and rheology. If the new solvent somehow decreases or increases these interactions, problems can result. Often more polar solvents like glycol ethers and alcohols can impact additives like ionic surfactants, associative thickeners, and polyelectrolyte dispersants.

Sometimes replacing a very water-insoluble solvent with a more water-soluble solvent can result in introducing water into the formulation. Sometimes even small amounts of additional water in solvents can cause problems in isocyanate cured coatings. Nonpolar solvents are generally not water soluble. Oxygenated solvents are usually more water-soluble than hydrocarbon solvents. Alcohols are more water-soluble than esters and higher molecular weight ketones are less water-soluble than low molecular weight ketones.

VOC regulations minimize the amount of solvent in the formulation. The lowest possible amount of solvent may be keeping everything dispersed until that critical moment of film formation. Even replacing a solvent used in a small amount can be a challenge. The unique dance that is the hydrogen bonding, electrostatic repulsions, and chain-chain interactions between polymers and additives can be upset by small changes in the solvent.

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Solvent Evaporation is a Dynamic Process

Because solvent evaporation is changing with time, there is a need for several different solvents in a formulation. Each solvent is responsible for a different part of the coating application or “drying” process. Solvent formulation can be thought of like movements of a symphony, with each solvent playing its part.

• The first movement contains the fast solvents that come out of the film and help determine how long it takes for the paint surface to no longer feel wet. This dry-to-touch time is important for handling the painted parts. For example, how many parts painted on an assembly line may be a function of how quickly the parts can be stacked or moved to other fabrication lines.


• The second movement is made up of the medium evaporating solvents. As they come out they help determine the film appearance and other application properties. Often the flow, leveling and other visual properties of the coatings are determined by the second movement. Many coating appearance problems can be overcome, not with the addition of expensive additives, but with proper selection of solvents during this movement.


• The final movement is made up of the slow solvents that may actually diffuse out of the polymer over several hours or even days. These remaining “tail” solvents have to be good solvents for the primary polymer in the coating formulation. Tail solvents with poor solvent activity for the coating resin can produce blushing problems. The wrong tail solvent can produce coating polymer-pigment destabilization during drying. It can negatively affect gloss and ultimate film performance. Often film integrity and strength are determined by how well these slow solvents perform. For example, if water-soluble solvents don’t diffuse out of the film, the coating can later be susceptible to water permeation into the polymer matrix causing corrosion and adhesion problems.

How these movements come together to make up the symphony performance determine the final properties and appearance of the final coating. That is why it is important to understand the dynamic process of solvent evaporation.

Other Solvent Properties that Can Affect Performance

• Density is important in solvent selection because of two important reasons:
  
  VOC Content – The calculation used world-wide in determining the level of VOC for coatings is based on either grams of VOC per liter or pounds of VOC per gallon. Minimizing these VOC density measurements for the coating gives lower density solvents an inherent advantage when calculating VOC. A solvent with a low density gives a higher volume concentration than a solvent with a high density. Low density solvents offer a significant advantage in meeting VOC requirements. Sometimes if a replacement solvent is found that has all the properties of the solvent you are replacing except for being higher density, the resulting VOC of the coating can be over the required minimum.
  
  Low Cost – Coating manufacturers usually purchase solvents by the pound, but then sell their formulated products by the gallon. Purchasing a low density solvent by the pound results in more gallons of solvent than purchasing a high density solvent. Using low density solvents usually translates to lower total raw material cost for the coating.


• Surface tension is also important in solvent selection because the solvents contribute greatly to the total surface tension of the coating during application.
  For a coating to wet out a substrate, the surface tension of the coating usually has to be lower than that of the substrate. When coating many plastic substrates, the selection of low surface tension solvents can assist in wetting and appearance. Problems of substrate wetting can occur when a solvent with low surface tension is replaced with a solvent of higher surface tension.


• Flash point of the solvent blend often determines how the coating can be handled, shipped, and labeled. The flash point is the lowest temperature at which the application of an ignition source will cause the vapor of a solvent to ignite. Be sure and replace a solvent with one of higher or equal flash point to ensure that the flammability labeling of your coating doesn’t change. The flash point of a solvent blend is usually closer to the lowest flash point solvent than the highest and should always be redetermined when making any solvent substitution.

Check the Regulations of the New Solvent

Solvents, because of being volatile and coming out of the coating and into the environment, are regulated more than any other coating ingredient. Solvent regulations are constantly changing world-wide. In addition, regulations sometimes differ depending on region, country to county, and district.

When making a solvent change in a coating formulation, be sure and determine if regulations in the location where the coating is to be sold, don’t restrict the sale of the new formulation.

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Conclusion

Replacing a solvent in a coating formulation is often a tedious and complicated process, requiring time and money.

• First try to determine what the solvent is doing in the formulation and how it is contributing to the coating application and final properties.
• Try to select a replacement solvent based on both evaporation rate and solvent activity. Often replacement of a solvent with a solvent blend allows easier matching of both evaporation rate and solvent activity.
  • If the solvent needing replacement is the active solvent for the primary resin in the formulation, replace it with an equally active solvent or solvent blend that has a similar evaporation rate.
  • Make sure that the replacement doesn’t increase the VOCs of the coating because of a higher density.
• Use the resources and expertise of your solvent manufacturers and the available online solvent reformulation computer programs.
• Be sure and check performance in the lab thoroughly before moving to production.
• Be aware that replacing any solvent may require other adjustments in the formulation to match required coating performance.
• Most importantly, before you start, make sure the solvent substitution is absolutely necessary.

Commercially Available Solvents Suitable for Paints and Coatings

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